The present invention relates to communications; and more particularly to signal conversion in wireless communication devices that service voice communications.
The number and popularity of wireless communications devices in use continues to rise rapidly all over the world. Not only have cellular telephones become very popular, but Wireless Local Area Networking (WLAN) devices have also proliferated. One standard for wireless networking, which has been widely accepted, is the Specification of the Bluetooth System, v. 1.0 (xe2x80x9cBluetooth Specificationxe2x80x9d). The Bluetooth Specification enables the creation of small personal area networks (PAN""s), where the typical operating range of a device is 100 meters or less. In a Bluetooth system, Bluetooth devices sharing a common channel sequence form a piconet. Two or more piconets co-located in the same area, with or without inter-piconet communications, is known as a scatternet.
The Bluetooth Specification supports voice communications between Bluetooth enabled devices. When a pair of Bluetooth devices support voice communication, the voice communications must be wirelessly supported in a continuous fashion so that carried voice signals are of an acceptable quality. Unexpected gaps, e.g., dropped packets, on the wireless link between supported Bluetooth devices causes degradation in the voice communication resulting in popping, static, or other unpleasant audible event. This problem is especially troublesome with Bluetooth devices since, in some operations, the communication link will regularly drop packets that carry the voice signals.
A further shortcoming of such operations relates to the manner in which packetized audio data is transmitted between Bluetooth devices. Consider an operation in which a first Bluetooth device transmits packetized audio data to a second Bluetooth device for presentation to a user. Because the Bluetooth WLAN supports data rates greatly in excess of those required for satisfactory voice service, each transmission from the first Bluetooth device carries a relatively large amount of packetized audio data. The duration of this transmission is typically small compared to the duration over which the second Bluetooth device will present the packetized audio data (carried in the transmission) to the user. Thus, the second Bluetooth device buffers the received packetized audio data and presents the packetized audio data (in a converted form) over an appropriate time period. However, if the packetized audio data stored in the input buffer is fully consumed prior to receipt of another transmission from the first Bluetooth device, it will appear to the second Bluetooth device that packetized audio data is lost (or severely delayed), and the second Bluetooth device will provided degraded audio to the serviced user.
Particular operational details occur during xe2x80x9cquietxe2x80x9d times in the operation of wireless devices servicing voice communications. In particular, Bluetooth (and other wireless) devices that service voice communications (via packetized audio data) include Analog to Digital Converters (ADCs) and Digital to Analog Converters (DACs). The ADC of the wireless device receives an analog audio signal from a coupled microphone and converts the analog audio signal to a digital audio signal. During this conversion process, the ADC may introduce xe2x80x9ctonesxe2x80x9d that are a byproduct of the sampling characteristics of the ADC. Likewise, the DAC receives a digital audio signal and converts the digital audio signal to an analog audio signal that it applies to coupled to a speaker. During this conversion process, the DAC may introduce xe2x80x9ctonesxe2x80x9d to the analog audio signal that are a byproduct of the conversion process. These xe2x80x9ctones,xe2x80x9d while having a relatively small magnitude compared to an active signal are noticeable during xe2x80x9cquietxe2x80x9d times. In the case of the ADC, xe2x80x9cquietxe2x80x9d times exist when no input is provided to the microphone. In the case of the DAC, xe2x80x9cquietxe2x80x9d times exist when data is lost or when incoming data contains no audio data.
Thus, there is a need for improved ADC and DAC operations for devices that serviced packetized voice communications.
In order to overcome the shortcomings of the prior Analog-to-Digital Converters (ADCs), an ADC constructed according to the present invention receives an analog signal and that converts the analog signal to digital data. The ADC includes a modulator, a decimation filter, and a time dither clock reduction circuit. The modulator receives the analog signal and a feedback signal and, based there upon, produces a modulated signal at a modulator clock rate. The decimation filter couples to the modulator, receives the modulated signal, and decimates and filters the modulated signal to produce the digital data. The time dither clock reduction circuit receives the modulated signal and provides the feedback signal to the modulator. The time dither clock reduction circuit applies both clock reduction and time dithering to the modulated signal to produce the feedback signal.
According to the present invention, at each modulator clock cycle, the time dithering clock reduction circuit considers modulated signals for a dithering factor, N, previous modulator clock cycles and a modulated signal for a current modulator clock cycle. If at least one constraint is satisfied for the N previous modulator clock cycles, the time dithering clock reduction circuit is allowed to transition the feedback signal with the modulated signal. If not, the time dithering clock reduction circuit holds the prior value of the feedback signal.
In a first particular operation, if the prior feedback signal is one, a sum of the modulated signals for the N previous modulator clocks is equal to N, the modulated signal for the current modulator clock is zero, and the time dithering clock reduction circuit transitions the feedback signal from one to zero. In a second particular operation, if the prior feedback signal is one, a sum of the modulated signals for the N previous modulator clocks is equal to N, the modulated signal for the current modulator clock is one, and the time dithering clock reduction circuit holds the feedback signal at one. In a third particular operation, if the prior feedback signal is zero, a sum of the modulated signals for the N previous modulator clocks is equal to zero, the modulated signal for the current modulator clock is one, and the time dithering clock reduction circuit transitions the feedback signal from zero to one. In a fourth particular operation, if the prior feedback signal is zero, a sum of the modulated signals for the N previous modulator clocks is equal to zero, the modulated signal for the current modulator clock is zero, and the time dithering clock reduction circuit holds the feedback signal at zero. These operations are varied slightly when the modulator is capable of producing more than two different outputs, e.g., xe2x88x921, 0, and 1.
After the time dithering clock reduction circuit is allowed to transition the feedback signal with the modulated signal, a new dithering factor, N, may be selected. In one particular operation for generating a new dithering factor, a random number is generated. The new dithering factor is based upon a comparison of the random number to at least one constraint.
The ADC of the present invention may be contained within a wireless local area network (WLAN) transceiving integrated circuit that services voice communications in a WLAN with at least one other WLAN device. The WLAN transceiving integrated circuit, in one embodiment, is formed as a single monolithic integrated circuit. Herein, the terms xe2x80x9caudio communicationsxe2x80x9d and xe2x80x9cvoice communicationsxe2x80x9d are both be used to refer to communications that contain information based upon audio signals that originate from or that are presented to a user in an audio format. Of course, the voice/audio communications need not be received directly from a human but may be generated by electronic equipment such as computers, media players, etc.
The WLAN transceiving integrated circuit may operate consistently with the Bluetooth Specification or with another standard, e.g., IEEE 802.11(a), IEEE 802.11(b), IEEE 802.11(c), etc. When the WLAN transceiving integrated circuit operates within a Bluetooth WLAN, the WLAN transceiving integrated circuit supports the Bluetooth Specification. In such case, the WLAN transceiving integrated circuit transmits packetized audio data to other Bluetooth devices and receives packetized audio data from other Bluetooth devices.
With the time dithering and clock reduction operations of the present invention, single modulator clock cycle resolution is maintained at the output of the time dither clock reduction circuit. Single modulator clock cycle resolution at the output of the ADC causes the modulator to better track the analog input signal. Further, by limiting the transitions of the time dither clock reduction circuit, power consumption of the ADC is reduced. Moreover, by adding time dithering, the ADC produces less output noise in the form of tones, i.e., frequencies corresponding to a fixed clock reduction operation. Reduction in output noise also lowers Electromagnetic Interference to address FCC and radio issues.
Other features and advantages of the present invention will become apparent from the following detailed description of the invention made with reference to the accompanying drawings.